Organized Turbulence Structures in the Near-Neutral Atmospheric Surface Layer

Abstract

The Tiederman method, which was originally designed for unambiguous detection of “bursts” in the buffer layer of smooth surface laboratory boundary-layer flow, is shown to work equally well in the neutral atmospheric surface layer. It enables estimation of characteristic timescales of significant structures responsible for the momentum transport: mean duration of and mean interval between bursts (upward transport of momentum deficit) and mean duration and interval between sweeps (downward transport of momentum excess). Data representing near neutral conditions from three field experiments over flat and homogeneous areas but with very different roughness characteristics are used in the analysis. For two low vegetation sites (i.e., the measuring heights are very much larger than the roughness sublayer height), it is found that all above-mentioned timescales are constant for any height and identical at the two sites. Scaling the results with the friction velocity indicates the corresponding scaling height to be proportional to the scaling height of the neutral boundary layer. The third site is a forest site, and the measurements were taken in the roughness sublayer. Here canopy inflection scaling is found to apply. The structures identified with the Tiederman method are shown to be responsible for more than 90% of the momentum flux, most of which is accomplished by a mean flow component within the structures. Results are discussed in light of direct numerical simulation results for fully turbulent but low Reynolds number flow over a dynamically smooth surface and large-eddy simulation results for a high Reynolds number case, and these show encouraging consistencies for the logarithmic layer. Turbulence production at the surface is, however, fundamentally different in the rough atmospheric case compared to the smooth case and is presumably governed by canopy wind profile inflection instability.